<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<script type="text/javascript">
function clearMap(){
    context.clearRect(0,0,300,200);
}
</script>
<body>
<canvas id="myCanvas" style="border:1px solid;" width="300" height="200">
</canvas>
<script type="text/javascript">
    var c=document.getElementById("myCanvas");
    var context=c.getContext("2d");
    context.strokeStyle="#FF00FF";
    context.beginPath();
    context.arc(200,150,100,-Math.PI*1/6,-Math.PI*5/6,true);
    context.stroke();
</script></br>
<input name="" type="button"  value="清空画布" onClick="clearMap();">
</body>
</html>
